Microshells of a diameter less than five milimeters can be formed by flowing a material in a molten state through an outer nozzle and another fluid such as gas through an inner nozzle to form a gas-filled pipe that breaks off into gas-filled shells. The shells fall into the top of a drop tower along which they are cooled to a solid state. If the shells are cooled rapidly, then the outside of the shell will not be precisely spherical, but will have a wavey surface. On the other hand, if the shell is cooled slowly, then the gas bubble within the shell will be off center.
One way to heat the shell material and the container and nozzle through which it passes, is by a resistance heater, either to heat the material to its molten temperature or to maintain it and the container at that temperature during shell formation. It is found that for high temperature-melting material, that the material tends to form unwanted lumps or particles of material. Also, microbubbles of gas in the material tend to coelesce to form bubbles of sufficient size to detract from the final spheres. A method for producing fluid-filled spheres which produced spheres with precisely spherical surfaces and gas bubbles that were precisely centered within the shell, and which avoided blockages of the nozzles by unwanted particles in the molten material which flows through the nozzles, would be of considerable value.